1. Field of the Invention
The present invention generally relates to earth boring drill bits used in the oil, gas and mining industries. More particularly, the present invention relates to tungsten carbide cutting elements having different lengths of insertion into supporting metal of a rotating cone drill bit.
2. Background Information
The success of rotary drilling enabled the discovery of deep oil and gas reservoirs. The rotary rock bit was an important invention that made rotary drilling economical. Only soft earthen formations could be commercially penetrated with the earlier drag bit, but the two-cone rock bit, invented by Howard R. Hughes, U.S. Pat. No. 930,759, drilled the hard caprock at the Spindletop Field near Beaumont, Tex., with relative ease. That venerable invention, in the early 1900""s, could drill a scant fraction of the depth and speed of the modern rotary rock bit. The original Hughes bit drilled for hours; the modem bit drills for days. Modem bits sometimes drill for thousands of feet instead of the mere few feet early bits drilled. Many advances have contributed to the impressive improvement of rotary rock bits.
In drilling boreholes in earthen formations by the rotary method, rock bits fitted with one, two, or three rolling cones or cutters are employed. The bit is secured to the lower end of a drillstring that is rotated from the surface or by downhole motors or turbines. The cones mounted on the bit roll and slide upon the bottom of the borehole as the drillstring is rotated, thereby engaging and disintegrating the formation material to be removed. The cones are provided with teeth or inserts that are forced to penetrate and gouge the bottom of the borehole by weight from the drillstring. The cuttings from the bottom and sidewalls of the borehole are carried to the surface in suspension by drilling fluid that is pumped down from the surface through the hollow, rotating drillstring. Certain aspects in the design of the rolling cones becomes particularly important if the bit is to penetrate deep into hard, high compressive strength, tough, and abrasive formation materials, such as limestones, dolomites and sandstones.
Because of the strength of these materials, insert penetration is reduced, and rock ribs form between the shallow craters generated by the inserts. Rock ribs formed in the high compressive strength, abrasive formation materials can become quite strong, causing the cone to ride up on the ribs and robbing the inserts of unit load necessary to accomplish effective penetration and crushing of formation material.
In hard and abrasive formations, the wear on the inserts, especially the heel inserts and the matrix holding them, is so severe that the inserts may eventually become dislodged from the cones, resulting in ring-outs on the gage. A loss of heel inserts leads to a ring-out on the gage because the gage inserts are forced to bear the entire burden of maintaining a minimum borehole diameter or gage, and the gage inserts cannot sustain this burden for long periods of drilling. This occurrence generates undesirable increases in lateral forces and torque on the cones, which lowers penetration rates and accelerates wear on the cone bearing and subsequent bit failure. The provision of cones with more closely spaced inserts reduces the size of rock ribs and the unit load on each individual insert, but it slows the rate of penetration and does not fundamentally change the wear characteristics at the tungsten carbide inserts and the steel matrix holding them.
Prior art earth-boring bits follow the conventional design rules, which use insert diameter to barrel length ratios, xe2x80x9cgrip ratios,xe2x80x9d in the 0.75 to 1.00 range to determine insert embedment. The limit for the barrel length of a cutting element is the minimum section of steel between adjacent inserts. The harder formation bits aim for maximum insert count with minimum section and therefore low grip ratios. In at least one instance in the past, grip ratios in the range from about 1.0 to 1.1 were used on inner rows of soft formation bits, having scoop-shaped inserts.
The bit body of the present invention has at least one cantilevered bearing shaft depending inwardly and downwardly from the bit body. A cone is mounted for rotation on the bearing shaft and includes a plurality of cutting elements arranged in generally circumferential rows on the cone. The rows of cutting elements include at least one heel row, at least one inner row, and at least one gage row. The cutting elements are formed of hard metal and are interference fit into apertures in the cone.
In one aspect of the invention, each heel row cutting element has at least one counterpart adjacent row cutting element that is spaced no farther from said heel row cutting element than any other adjacent row cutting element. The two neighboring cutting elements may be considered a proximal pair, although they are normally different in size and shape. Also, each heel row cutting element may be paired with more than one adjacent row cutting element, because normally there will be two adjacent row cutting elements spaced the same distance from each heel row cutting element. One of the cutting elements within some of the proximal pairs has a longer barrel length and greater grip ratio than the other cutting element within the same proximal pair. However, none of the proximal pairs has two cutting elements with the longer barrel lengths. This assures a minimum section of supporting metal in the cone body between the cutting elements.
In another aspect of the invention, one of the heel rows has cutting elements with more than one grip ratio that alternate with each other in a selected pattern. In a first pattern, a greater grip ratio cutting element alternates with a lesser grip ratio cutting element. The adjacent row will have lesser grip ratios. This arrangement is utilized on the cone that has a greater density in the heel row than the other cones. The other cones of this embodiment may have all greater grip ratio heel row cutting elements and all lesser grip ratio adjacent row cutting elements.
In another embodiment, the pattern for all of the cones comprises two lesser grip ratio cutting elements separated by one greater grip ratio cutting element. This is employed in both the heel and adjacent rows. A third embodiment employs heel row cutting elements with standard grip ratios. The adjacent cutting elements, however, will be of greater grip ratios, or alternating with greater and lesser grip ratios.